linux-kernel - RE: [PATCH v3 2/7] Drivers: hv: Enable VTL mode for arm64

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Date: Mon, 5 Aug 2024 19:51:36 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Roman Kisel <romank@...ux.microsoft.com>, Saurabh Singh Sengar
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Subject: RE: [PATCH v3 2/7] Drivers: hv: Enable VTL mode for arm64

From: Roman Kisel <romank@...ux.microsoft.com> Sent: Monday, August 5, 2024 8:24 AM
> 
> On 8/4/2024 9:05 PM, Saurabh Singh Sengar wrote:
> > On Mon, Aug 05, 2024 at 03:01:58AM +0000, Michael Kelley wrote:
> >> From: Roman Kisel <romank@...ux.microsoft.com> Sent: Friday, July 26, 2024 3:59
> PM
> >>>
> >>> Kconfig dependencies for arm64 guests on Hyper-V require that be ACPI enabled,
> >>> and limit VTL mode to x86/x64. To enable VTL mode on arm64 as well, update the
> >>> dependencies. Since VTL mode requires DeviceTree instead of ACPI, don't require
> >>> arm64 guests on Hyper-V to have ACPI.
> >>>
> >>> Signed-off-by: Roman Kisel <romank@...ux.microsoft.com>
> >>> ---
> >>>   drivers/hv/Kconfig | 6 +++---
> >>>   1 file changed, 3 insertions(+), 3 deletions(-)
> >>>
> >>> diff --git a/drivers/hv/Kconfig b/drivers/hv/Kconfig
> >>> index 862c47b191af..a5cd1365e248 100644
> >>> --- a/drivers/hv/Kconfig
> >>> +++ b/drivers/hv/Kconfig
> >>> @@ -5,7 +5,7 @@ menu "Microsoft Hyper-V guest support"
> >>>   config HYPERV
> >>>   	tristate "Microsoft Hyper-V client drivers"
> >>>   	depends on (X86 && X86_LOCAL_APIC && HYPERVISOR_GUEST) \
> >>> -		|| (ACPI && ARM64 && !CPU_BIG_ENDIAN)
> >>> +		|| (ARM64 && !CPU_BIG_ENDIAN)
> >>>   	select PARAVIRT
> >>>   	select X86_HV_CALLBACK_VECTOR if X86
> >>>   	select OF_EARLY_FLATTREE if OF
> >>> @@ -15,7 +15,7 @@ config HYPERV
> >>>
> >>>   config HYPERV_VTL_MODE
> >>>   	bool "Enable Linux to boot in VTL context"
> >>> -	depends on X86_64 && HYPERV
> >>> +	depends on HYPERV
> >>>   	depends on SMP
> >>>   	default n
> >>>   	help
> >>> @@ -31,7 +31,7 @@ config HYPERV_VTL_MODE
> >>>
> >>>   	  Select this option to build a Linux kernel to run at a VTL other than
> >>>   	  the normal VTL0, which currently is only VTL2.  This option
> >>> -	  initializes the x86 platform for VTL2, and adds the ability to boot
> >>> +	  initializes the kernel to run in VTL2, and adds the ability to boot
> >>>   	  secondary CPUs directly into 64-bit context as required for VTLs other
> >>>   	  than 0.  A kernel built with this option must run at VTL2, and will
> >>>   	  not run as a normal guest.
> >>> --
> >>> 2.34.1
> >>>
> >>
> >> In v2 of this patch, I suggested [1] making a couple additional minor changes
> >> so that kernels built *without* HYPER_VTL_MODE would still require
> >> ACPI.  Did that suggestion not work out?  If that's the case, I'm curious
> >> about what goes wrong.
> >
> > Hi Michael/Roman,
> > I was considering making HYPERV_VTL_MODE depend on CONFIG_OF. That should
> address
> > above concern as well. Do you see any potential issue with it.
> >
> Michael,
> 
> I ran into a pretty gnarly recursive dependencies which in all fairness
> might stem from not being fluent enough in the Kconfig language. Any
> help of how to approach implementing your idea would be greatly appreciated!
> 

This is what I had in mind:

--- a/drivers/hv/Kconfig
+++ b/drivers/hv/Kconfig
@@ -5,7 +5,8 @@ menu "Microsoft Hyper-V guest support"
 config HYPERV
        tristate "Microsoft Hyper-V client drivers"
        depends on (X86 && X86_LOCAL_APIC && HYPERVISOR_GUEST) \
-               || (ACPI && ARM64 && !CPU_BIG_ENDIAN)
+               || (ARM64 && !CPU_BIG_ENDIAN)
+       depends on (ACPI || HYPERV_VTL_MODE)
        select PARAVIRT
        select X86_HV_CALLBACK_VECTOR if X86
        select OF_EARLY_FLATTREE if OF
@@ -15,7 +16,7 @@ config HYPERV

 config HYPERV_VTL_MODE
        bool "Enable Linux to boot in VTL context"
-       depends on X86_64 && HYPERV
+       depends on X86_64
        depends on SMP
        default n
        help

HYPERV_VTL_MODE can now be selected independently of HYPERV.
The existing code should be such that even if someone is building a
random config and gets HYPERV_VTL_MODE without HYPERV, the
kernel will build and run in a non-Hyper-V environment and isn't
broken somehow.

For HYPERV to be selected, either ACPI must already be selected, or
HYPERV_VTL_MODE must already be selected. So "normal" kernels are
still enforced to require ACPI. But if building with HYPERV_VTL_MODE,
then ACPI is optional.

Saurabh's idea of adding "depends on OF" to HYPERV_VTL_MODE
should also work with these changes.

I haven't fully tested the above with all the relevant combinations
on both x86 and arm64, but I think the logic makes sense.

Michael